1. Technical Field
The present invention relates to an image pickup device and an image pickup apparatus which can read out electric charges from a photoelectric conversion element in a destructive read-out method and a nondestructive read-out method.
2. Related Art
As one of problems of an image pickup apparatus using a solid-state image pickup device represented by a digital still camera, narrowness of the dynamic range is exemplified. Therefore, an image acquired by photographing a scene having a significantly large contrast includes an over exposure (saturation of pixels) in a high-intensity area and blackened display in a low-intensity area.
A number of methods are proposed in order to solve the above-described problem. As one of these methods, there is a method of combining two (or a plurality of) images having different exposure values. Known examples in which this method is employed include an image pickup apparatus described in JP-A-2004-32583.
The image pickup apparatus in this document is an image pickup apparatus in which standard image signals having adequate exposure values and non-standard image signals having smaller exposure value than the standard image signals are obtained by exposure value variable means which enables photographing with varied exposure values, characterized by means for generating saturation determination signals (mask image signals in the invention) which indicates whether or not the value of the outputted standard image signal exceeds a threshold value (for determining saturation of pixels) and means for switching two image signals on the basis of the saturation determination signal (for replacing the saturated pixels with the standard image signals by values of the nonstandard image signal. Accordingly, images having a wide dynamic range may be acquired.
However, in the related art described in JP-A-2004-32583, when saturation determination signals (saturated pixels are represented by “1”, and non-saturated pixels are represented by “0”) are generated from the standard image signals having noise added thereto, and the images are combined on the basis of the determination signals, the quality of the combined image may be degraded. More specifically, the image pickup device includes more dark current noise with elapse of time and subjects to more influence of the noise. Variations in sensitivity or saturation from pixel to pixel also result in erroneous determination. For example, as shown in FIG. 15, it occurs because there appear black pixels here and there even though all the pixels are to be originally white pixels due to variations from pixel to pixel. When the images are combined using such saturation determination signals, the standard image signals and the non-standard image signals are mixed falsely (all of them must originally be the non-standard image signals). Although this problem may be solved by changing the threshold value according to variations in characteristics of the respective pixels, a significant labor to set the threshold value for each pixel is required.
On the other hand, there is also a method of acquiring an image having a wide dynamic range by adding the standard image signals and the non-standard image signals (FIG. 16). This simple addition is referred to as Knee system. The Knee system does not include level determination of pixel value, and hence the problem in the related art as in JP-A-2004-32583 does not occur. Since the Knee system includes level compression of a high-intensity area, there arises a problem such that tone reproduction in the high-intensity area is impaired and linearity is disappeared in the device which can display a wide dynamic range (for example, the device which has a capability of contrast display of, for example, 10000:1), so that reproduction of the real colors are not achieved (in the case of a sensor in which a color filter is mounted).
As another problem, when the standard exposed image and the non-standard exposed image are taken at separate timings twice and are combined, a blurred image is resulted.